We present a thorough analysis of the signal line shapes of Fouriertransform-based noise-immune cavity-enhanced optical frequency combspectroscopy (NICE-OFCS). We discuss the signal dependence on the ratio of themodulation frequency, f${_m}$, to the molecular line width, {\Gamma}. Wecompare a full model of the signals and a simplified absorption-like analyticalmodel that has high accuracy for low f${_m}$/{\Gamma} ratios and is much fasterto compute. We verify the theory experimentally by measuring and fittingNICE-OFCS spectra of CO${_2}$ at 1575 nm using a system based on an Er:fiberfemtosecond laser and a cavity with a finesse of ~11000.
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